A wide range of 2-dimensional (2-D) atomic crystals exist in nature. The simplest and most studied is graphene (an atomic-scale 2-D honeycomb lattice of carbon atoms), followed by Boron Nitride (BN). However, hundreds more exist including transition metal dichalcogenides (TMDs) such as Molybdenum disulphide (MoS2), Niobium diselenide (NbSe2), Vanadium telluride (VTe2), transmission metal oxides such as Manganese dioxide (MnO2) and other layered compounds such as Antimony telluride (Sb2Te3), Bismuth telluride (Bi2Te3). Depending on the exact atomic arrangement, these crystals can be metals, insulators or semiconductors.
Layered materials, come in many varieties with one family having the formula MXn (where M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, Mo, W, Tc, Re, Ni, Pd, Pt, Fe, Ru; X═O, S, Se, Te; and 1≤n≤3). A common group are the transition metal dichalcogenides (TMDs) which consist of hexagonal layers of metal atoms sandwiched between two layers of chalcogen atoms. While the bonding within these tri-layer sheets is covalent, adjacent sheets within a TMD crystal are weakly bound by van der Waals interactions. Depending on the co-ordination and oxidation state of the metal atoms, TMDs can be metallic or semiconducting. For example, Tungsten disulphide (WS2) is a semiconductor while Tantalum disulphide (TaS2) and Platinum telluride (PtTe2) are metals. This versatility makes them potentially useful in many areas of electronics.
Over the last decade graphene has become the most studied of all nanomaterials due to its wide range of useful properties.1 A graphene sheet consists of an atomically thin array of sp2 bonded carbon atoms organized in a planar hexagonal arrangement and was first produced and exploited in 2004 by Geim and Novosolov. However, they were only able to produce individual sheets of graphene by micromechanical cleavage of graphite.2 
The novel electronic properties of graphene have been well documented.1 In addition, graphene is ideal for a range of applications. For example, it is the strongest material known to man3, has been fabricated into large area transparent conductors4 and is extremely promising in the fields of composites, coatings and electronic devices. Because of these exciting properties, a number of new methods of graphene production have been developed such as by annealing SiC substrates5 or growth on metal supports.6 
These methods for producing graphene have been very successful to date. However, it is likely that many future industrial applications of graphene will be in sectors such as large-area coatings or composite fillers which require the production of graphene in very large quantities.7 No methods currently exist which are scalable and give defect free graphene in large quantities. To this end, it is likely that liquid phase production methods will be required.8 
Over the last number of years, many methods have been demonstrated to produce graphene (and more recently other 2-D materials) in reasonable quantities. The two main methods are chemical vapour deposition (CVD) and liquid exfoliation. CVD is a method that can be used to grow monolayers of graphene4 or other 2D materials such as MoS29 on surfaces, primarily for electronic applications. The typical masses deposited are ˜10−7 kg/m2. However, many applications, such as the use of graphene as a filler in composites will require much larger masses, potentially many tonnes per year. In addition, the form of CVD graphene—a monolayer on a surface—is unsuitable for applications such as composites or porous electrodes. It is widely accepted that liquid exfoliation is the only method to produce graphene in a versatile form (micron sized flakes) in large quantities.
As discussed, graphite is just one member of a large family of layered crystals. The basic building block of such a crystal is an atomically thin sheet of material, named graphene in the case of graphite. These “nanosheets” are stacked on top of each other and bound by van der Waals forces. Between atoms or molecules, these forces are relatively weak. However, when integrated over the areas of nanosheets, these forces can be quite large. This makes it difficult to separate (or exfoliate) the nanosheets from their parent crystal. The most promising methods to do this tend to operate in a liquid environment—liquid exfoliation.
The most common method of exfoliating graphene is to oxidise graphite to create graphite oxide. Here oxygen containing groups are covalently bound to the graphene. This swells the crystal, weakening the binding energy between layers. It also allows water to intercalate between the layers which further weakens the binding, ultimately allowing exfoliation10. The oxide groups can be removed by reduction either chemically or thermally11. The problem is that the graphene produced by this method is very defective. It always contains missing atoms or even holes in the nanosheets which severely distorts its mechanical and electrical properties to the extent that it cannot be considered graphene but only graphene-like. Thus, oxidisation cannot be used to develop a simple scalable method to produce defect free graphene.
Another method is based on intercalation of species such as ions between the layers of the crystal and has been widely applied to exfoliate layered materials including graphite12, and MoS213. Intercalation, often of ionic species, increases the layer spacing, weakening the interlayer adhesion, and reducing the energy barrier to exfoliation. Intercalants such as n-butyllithium13 or IBr12 can transfer charge to the layers, resulting in a further reduction of interlayer binding. Subsequent treatment such as thermal shock12 or ultrasonication13 in a liquid completes the exfoliation process. The exfoliated nanosheets can be stabilised electrostatically by a surface charge13b or by surfactant addition12. In the case of MoS2, this method tends to give highly exfoliated nanosheets but has drawbacks associated with its sensitivity to ambient conditions13a. The real disadvantage here is that this process contains multiple steps (intercalation followed by exfoliation). Crucially, the intercalation step is slow, sensitive to ambient conditions and not scalable. Thus, intercalation cannot be used to develop a simple, scalable method to produce defect free graphene (or other 2D materials).
Another method has been developed by one of the inventors. It involves the ultrasonication of a layered crystal such as graphite14 or MoS2 in a suitable solvent15 or aqueous surfactant solution16. Here the high level of ultrasonic power (˜300 W) being dissipated in a small volume of liquid (˜100 ml) results in a very high power density (˜3000 W/L). The energy dissipated acts to break up the crystal into individual nanosheets. However, this process cannot give true exfoliation unless the nanosheets are stabilised against reaggregation. This is achieved either by choosing special solvents which stabilise the exfoliated nanosheets by interacting with their surface14-15 or by sonicating in a water-surfactant or water-polymer mixture. The surfactant molecules (or ions in some cases) or polymer chains stick to the nanosheets surface stabilising them against reaggregation. This method has considerable advantages as it is known to produce defect free graphene in one step. The problem with this method is the high energy density required for ultrasonic exfoliation. Using typical ultrasonic processors, high power densities can only be achieved in small liquid volumes. This means that the only way to scale-up the process is to increase the number of processors used. Thus the cost scales linearly with the amount of graphene (or other 2-D nanosheets) produced. Thus, this method cannot achieve any economies of scale and so is not a candidate as a simple scalable method to produce defect free graphene.
International Pamphlet Publication No. WO 2011/014347 A1 mentions the use of shear mixing, but only when outlining alternative approaches involving intercalation or graphite oxide routes. Chinese Patent Application No. CN 101671015 A describes a process that involves the use of ball milling, followed by a sonication step. Similarly, Chinese Patent Application No. CN 102583350 A describes processing a graphite-liquid mixture in a “gear group” which is used to feed the mixture into a sonication step. Furthermore, UK Patent Application No. GB 2483288 A describes a process for exfoliating layered materials from the bulk crystal. The process described used sonication only for exfoliation in water/surfactant solution.
Another exfoliation method that has been used is ion intercalation followed by shear mixing. In this method, ions are intercalated between the layers of the layered crystal. As described above, intercalation increases the layer spacing, weakening the interlayer adhesion, and reducing the energy barrier to exfoliation. This is a critically important step. This weakening of the forces binding the layers together is generally thought to be critical to facilitate exfoliation. Once this has been achieved, the swelled layered crystallites e.g. vermiculite17, TaS218, graphite (U.S. Pat. Nos. 5,186,919; 8,132,746), can be exfoliated using a process called shear mixing. In this process, an impellor or rotor/stator combination rotates at high speed in the liquid containing the layered crystal. This results in turbulent flow which can act to exfoliate the layered crystal. The main advantage of this technique is mixing using impellors or rotor/stators is known to be potentially scalable to industrial levels (depending on the components being mixed). However, there is a serious drawback. The requirement that ion intercalation must be carried out to weaken the bonds between layers means that the process is neither simple (not one-step) nor scalable (the intercalation process is slow, sensitive to ambient conditions and not easily or cheaply scalable). In addition, the presence of residual ions may degrade the properties of the graphene and so effectively acts as a contaminant. This pre-treatment can be time consuming, expensive, require special reaction conditions and limit the possibility for industrial scale-up.
It is worth considering whether the intercalation step is required. For a standard bench-top high-shear mixer (e.g. the Silverson L5M) the maximum power output is ˜250 W. These are typically used to mix ˜liter sized volumes (mixing is inefficient at low volumes). This means the dissipated power density is <250 W/L. This is a factor of ˜10 below the figure quoted above for ultrasonication. Thus, the received wisdom would imply that shear mixing should not be powerful enough to break the bonds connecting the nanosheets in layered crystals unless these bonds have been weakened by a process such as intercalation. Thus, because of the limits associated with ion intercalation, ion intercalation coupled with shear mixing is not a candidate as a simple scalable method to produce defect free graphene or other nanosheets.
There is one paper that described the exfoliation of graphite to give graphene using shear mixing17. Alhassan and co-workers used a stirred impeller style of mixer with turbulent flow to attempt to exfoliate graphene in water and laponite, an additive which has rapid gelation kinetics. They showed that if water or water and surfactant solution in the absence of laponite are used then the graphene rapidly aggregates and sediments. While the authors do show evidence of graphene exfoliation, they note that in the absence of a stabilising solvent or surfactant, aggregation and sedimentation of the graphitic material will occur. As such the message from this paper is that graphene cannot be made by shear mixing of graphite in liquids. In fact the only way aggregation could be prevented was by the addition of laponite clay (these are planar nanoparticles which can be exfoliated in water), which sets as a solid gel when added to water. There is no observation of exfoliated graphene in the absence of laponite. Mixing was always carried out in the presence of laponite.
Laponite is a clay which consists of charged 2-dimensional nanosheets. The charge is compensated by mobile counterions which exist between the layers. As with ion exfoliation, these counterions mean that laponite should be easily exfoliated in water using a shear mixer. On exfoliation, the mobile counterions will be distributed throughout the liquid, especially at the high clay concentrations used in this work. This means that there are many ionic species available to intercalate between the graphene layers, thus weakening the interlayer interaction and facilitating exfoliation. Thus, it is likely that the presence of laponite is necessary to allow graphite exfoliation. The exfoliated graphene is kinetically stabilised against reaggregation by being embedded in solid polymer or by adding gelling clay material (see U.S. Pat. No. 7,906,053 and (14), respectively).
The problem here is that once the graphene is mixed with the clay, it is subsequently un-recoverable. Thus, this method, although it produced graphene, cannot be used as a graphene production method and it is probable that the graphene cannot be produced in the absence of the clay. Thus, clay addition, prevents both processing and collection of the exfoliated graphene.
It is an object of the present invention to overcome at least one of the above-mentioned problems.